Thiazole-based semiconductor compound and organic thin film transistor using the same

ABSTRACT

Provided are an organic semiconductor compound using thiazole, and an organic thin film transistor having an organic semiconductor layer formed of the organic semiconductor compound using thiazole. The novel organic semiconductor compound including thiazole has liquid crystallinity and excellent thermal stability, and thus is provided to form an organic semiconductor layer in the organic thin film transistor. To this end, a silicon oxide layer is formed on a silicon substrate, and an organic semiconductor layer including thiazole is formed on the silicon oxide layer. In addition, source and drain electrodes are formed on both edge portions of the organic semiconductor layer. The organic thin film transistor using the organic semiconductor layer has an improved on/off ratio and excellent thermal stability. Also, a solution process can be applied in its manufacture.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No.2007-0123143, filed on Nov. 30, 2007, the disclosure of which is herebyincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic semiconductor compound andan organic thin film transistor using the same, and more particularly,to an organic semiconductor compound including thiazole, and an organicthin film transistor using the same which can be applied in a solutionprocess and have an improved on/off ratio.

2. Description of the Related Art

Organic thin film transistors have several advantages which have madethem a focus of active research and development. In particular, anorganic thin film transistor can be fabricated in a simple process andat low cost, and can be easily applied to a flexible electronic circuitsubstrate that is resistant to breakage due to impact, and can be bentand folded.

In addition, compared to a conventional thin film transistor usingamorphous silicon and polysilicon, an organic thin film transistor canbe fabricated in a simple process and at low cost, and has excellentcompatibility with substrates on which other electronic parts aremounted. It is for these reasons that organic thin film transistors havebeen widely researched in recent times.

However, for an organic thin film transistor to perform well, it shouldhave a high on/off ratio and high charge mobility.

Also, an organic semiconductor forming the organic thin film transistormay be classified as small molecule semiconductor or polymersemiconductor depending on its molecular weight.

Generally, small molecule semiconductor can be easily purified to almostperfectly remove impurities and thereby obtain excellent electricalproperties. But, it cannot be applied in a solution process such as spincoating and printing, and thus a thin film should be formed by vacuumdeposition using an expensive vacuum device.

On the other hand, polymer semiconductor is difficult to thoroughlypurify and its impurities cannot be perfectly removed. But, it has highthermal stability and can be applied in a solution process such spincoating and printing, and this it is preferable for low-cost processingand mass production.

Therefore, there is need to develop an organic semiconductor compoundsimultaneously having the advantages of both small moleculesemiconductor and polymer semiconductor.

SUMMARY OF THE INVENTION

The present invention is directed to an organic semiconductor compoundthat has an improved on/off ratio and excellent thermal stability, andcan be used in an organic thin film transistor to which a solutionprocess can be applied.

The present invention is also directed to a method of fabricating anorganic thin film transistor using the organic semiconductor compound.

The present invention is also directed to an organic thin filmtransistor using the organic semiconductor compound.

According to an embodiment of the present invention, a thiazole-basedorganic semiconductor compound is represented by Formula 1 below:

Here, R1, R2, R3 and R4 are independently selected from a groupconsisting of hydrogen, a C1-C25 alkyl group, a C1-C25 alkoxy group, anda C6-C30 aryl group substituted with C1-C25 alkyl and alkoxy groups.

In Formula 1, X and Y are substituents or substituent groups identicallyor independently selected from a group consisting of

wherein Z1 is an independently single- or multi-substituted substituentselected from a group consisting of hydrogen, a hydroxyl group, asubstituted or unsubstituted methyl group, a carbonyl group, an aminogroup, a substituted or unsubstituted C1-C30 alkyl amino group, asubstituted or unsubstituted C1-C30 aryl amino group, a substituted orunsubstituted heteroaryl amino group, a cyano group, a halogen atom, asubstituted or unsubstituted C1-C30 alkyl group, a substituted orunsubstituted C3-C30 cycloalkyl group, a substituted or unsubstitutedC1-C30 alkoxy group, a substituted or unsubstituted C6-C30 aryl group, asubstituted or unsubstituted C6-C30 aryl alkyl group, a substituted orunsubstituted C2-C30 heteroaryl group, and a substituted orunsubstituted C2-C30 heterocyclic group.

R5, R6, R7 and R8 are selected from a group consisting of hydrogen, aC1-C25 ethylene oxide group, a C1-C25 alkyl group, a C1-C25 fluorinatedalkyl group, a C1-C25 alkoxy group, a C1-C25 alkylene oxide group, aC1-C25 polydimethylsiloxane group, a C1-C25 alkylene sulfide group, anda C1-C25 ethylene sulfide group.

According to another embodiment of the present invention, a method offabricating an organic thin film transistor includes: preparing asilicon substrate; forming an insulating layer using silicon oxide onthe silicon substrate; forming an organic semiconductor layer using acompound of Formula 1 on the insulating layer; and forming source anddrain electrodes on the organic semiconductor layer.

Here, the organic semiconductor layer may be formed by depositing thematerial of Formula 1 under high vacuum.

Alternatively, the organic semiconductor layer may be formed by coatingthe material of Formula 1 dissolved in a solution.

According to still another embodiment of the present invention, anorganic thin film transistor includes: a silicon substrate; aninsulating layer formed of silicon oxide on the silicon substrate; anorganic semiconductor layer formed of the material of Formula 1 on theinsulating layer; and electrode layers formed on the organicsemiconductor layer.

Here, the organic semiconductor layer may be formed by depositing thematerial of Formula 1 under high vacuum.

Alternatively, the organic semiconductor layer may be formed by coatingthe material of Formula 1 dissolved in a solution.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other objects, aspects and advantages of the invention willbecome apparent and more readily appreciated from the followingdescription of exemplary embodiments and the accompanying drawings, inwhich:

FIG. 1 is a 1H-NMR graph of an organic semiconductor compoundrepresented by Formula 2 of the present invention;

FIG. 2 is a thermogravimetric graph of the organic semiconductorcompound represented by Formula 2 of the present invention;

FIG. 3 is a differential scanning calorimetry graph of the organicsemiconductor compound represented by Formula 2 of the presentinvention;

FIG. 4 is an x-ray diffraction graph of the organic semiconductorcompound represented by Formula 2 of the present invention;

FIG. 5 is a graph showing electrical properties of an organic thin filmtransistor device using the organic semiconductor compound representedby Formula 2 of the present invention; and

FIG. 6 is a cross-sectional view of an organic thin film transistorhaving an organic semiconductor layer formed of the organicsemiconductor compound represented by Formula 1.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to exemplary embodiments of thepresent invention, examples of which are shown in the accompanyingdrawings.

However, it should be understood that the present invention is notlimited to the described exemplary embodiments, but includes variousmodifications, equivalents and alternatives.

In this description, the terms “first” and “second” may be used todistinguish similar elements from one another, however these termsshould not be construed as limiting the elements in any way.

Hereinafter, exemplary embodiments of the present invention will bedescribed in more detail with reference to the accompanying drawings.When the same element appears in different drawings, it will always bedenoted by the same reference numeral and it will only be describedonce. It should be clear that these embodiments are provided only toexplain the present invention, not to limit its scope.

A thiazole-based organic semiconductor compound according to the presentinvention is represented by Formula 1:

In Formula 1, R1, R2, R3 and R4 are identically or independentlyselected from a group consisting of hydrogen, a C1-C25 alkyl group, aC1-C25 alkoxy group, and a C6-C30 aryl group substituted with C1-C25alkyl and alkoxy groups.

In Formula 1, X and Y are substituents or substituent groups identicallyor independently selected from a group consisting of

wherein Z1 is an independently single- or multi-substituted substituentselected from a group consisting of hydrogen, a hydroxyl group, asubstituted or unsubstituted methyl group, a carbonyl group, an aminogroup, a substituted or unsubstituted C1-C30 alkyl amino group, asubstituted or unsubstituted C1-C30 aryl amino group, a substituted orunsubstituted heteroaryl amino group, a cyano group, a halogen atom, asubstituted or unsubstituted C1-C30 alkyl group, a substituted orunsubstituted C3-C30 cycloalkyl group, a substituted or unsubstitutedC1-C30 alkoxy group, a substituted or unsubstituted C6-C30 aryl group, asubstituted or unsubstituted C6-C30 aryl alkyl group, a substituted orunsubstituted C2-C30 heteroaryl group, and a substituted orunsubstituted C2-C30 heterocyclic group.

R5, R6, R7 and R8 are independently selected from a group consisting ofhydrogen, a C1-C25 ethylene oxide group, a C1-C25 alkyl group, a C1-C25fluorinated alkyl group, a C1-C25 alkoxy group, a C1-C25 alkylene oxidegroup, a C1-C25 polydimethylsiloxane group, a C1-C25 alkylene sulfidegroup, and a C1-C25 ethylene sulfide group.

EXAMPLE 1 Synthesis of Compound of Formula 2

A compound of Formula 2 was synthesized according to Scheme 1. Thecompound of Formula 2 is an example of Formula 1. A compound (1) iscommercially available from the Aldrich Company.

200 ml of THF dried over sodium was cooled to 0° C., and then 15.18 ml(138 mmol) of TiCl₄ was slowly added to the THF while stirring. After 15minutes, 18 g (276 mmol) of zinc was added, and then the resultingmixture was heated while stirring for 1 hour. After the mixture wascooled to 0° C., 13 g (115.92 mmol) of the compound (1) was dissolved in50 ml of the dried THF, and the resulting mixture was heated whilestirring for 3 hours. After that, the mixture was slowly cooled to roomtemperature, and 50 ml of ice water was added to terminate the reaction.Precipitates were filtered and recrystallized using cyclone hexane toobtain a compound (2) (yield: 65%).

14 ml of DMF was stirred to be well mixed with 1.5 ml (16.5 mmol) ofPOCl₃ at 0° C., and the resulting mixture was slowly added whilestirring to a solution made from dissolving 2.74 g (14.25 mmol) of thecompound (2) in 40 ml of DMF at 0° C. After that, the resulting solutionwas slowly heated to room temperature and stirred at 50° C. for 3 hours.The resulting solution was neutralized using 100 ml of 1M NaOH aqueoussolution, extracted using benzene, and then purified by a silica columnapparatus to obtain a compound (3) (yield: 70.08%).

2.20 g (10 mmol) of the compound (3) and 0.541 g (4.5 mmol) ofdithiooxamide were dissolved in 20 ml of 1,2-dichlorobenzene and stirredand heated for 24 hours. Afterwards, the solution was cooled to roomtemperature, precipitated using 100 ml of ethanol, and filtered. Thefiltered material was washed with ethanol several times and purified bya silica column apparatus to obtain the compound of Formula 2 (yield:22%).

A structure of the compound of Formula 2 was analyzed by 1H-NMR shown inFIG. 1 and mass spectrometry. Referring to FIG. 2, which showsthermogravimetric analysis results of the synthesized compound ofFormula 2, 5% mass reduction of the synthesized compound of Formula 2 isdetected at 302° C. This indicates that the synthesized compound ofFormula 2 has excellent thermal stability. Referring to FIG. 3, whichshows differential scanning calorimetry results, the synthesizedcompound of Formula 2 has phase change points at 155° C. and 240° C. InFIG. 3, which shows the differential scanning calorimetry results, twodifferent phase change temperatures are observed, which indirectlyindicates that the compound of Formula 2 has liquid crystallinity.Referring to FIG. 4, which shows an X-ray diffraction graph of anorganic semiconductor represented by Formula 2, it can be noted that theorganic semiconductor represented by Formula 2 has 5 peaks, whichindicates that it has crystallinity.

A method of fabricating an organic thin film transistor having anorganic semiconductor layer formed of the compound of Formula 1according to the present invention includes forming a silicon substrate,forming an insulating layer using silicon dioxide, forming an organicsemiconductor layer using the compound of Formula 1, and forming sourceand drain electrodes.

EXAMPLE 2 Fabrication of Organic Thin Film Transistor

The following description concerns an example of a method of fabricatingan organic thin film transistor having an organic semiconductor layerformed of the compound of Formula 2, which is an example of the compoundof Formula 1.

First, a silicon substrate 110 is provided. The silicon substrate 110may be doped with an n-type dopant. In addition, the n-type siliconsubstrate serves as a gate electrode of an organic thin film transistorto be formed later.

Subsequently, an insulating layer 120 was formed by thermally growingsilicon oxide to a thickness of 300 nm on the substrate. The insulatinglayer 120 may be formed by various methods except for thermal growth.That is, the insulating layer 120 may be formed by chemical vapordeposition, physical vapor deposition or atomic layer deposition.

Subsequently, the compound of Formula 2 was deposited under vacuum, ordissolved in a solution such as chlorobenzene and then spin-coated,thereby forming an organic semiconductor layer 130 to a thickness of 50nm.

Finally, 30 ml of gold was deposited on the organic semiconductor layer130 under vacuum, thereby forming source and drain electrodes 140.

Electrical properties of the organic thin film transistor fabricated bythe above-described process was analyzed as shown in FIG. 5 using anapparatus for analyzing semiconductor characteristics. Referring to FIG.5, the organic thin film transistor of this Example exhibits anexcellent on/off ratio of more than 10⁶.

FIG. 6 is a cross-sectional view of an organic thin film transistorhaving an organic semiconductor layer formed of the compound of Formula1.

Referring to FIG. 6, the organic thin film transistor of this Example isformed of a silicon substrate 110, an insulating layer 120, an organicsemiconductor layer 130 and to electrode layers 140.

The insulating layer 120 is formed on the silicon substrate 110 servingas a gate electrode. The insulating layer 120 is formed of siliconoxide, and the organic semiconductor layer 130 is formed on theinsulating layer 120. The organic semiconductor layer 130 is formed ofthe organic compound represented by Formula 2. In addition, theelectrode layers 140 are formed of a metallic material on edge portionsof the organic semiconductor layer 130. The electrode layers serve assource and drain electrodes, respectively. Preferably, the electrodelayers include gold. However, it can be clearly understood by thoseskilled in the art that any one of the conductors formed on the organicsemiconductor layer by an appropriate method can be used for anelectrode layer.

The substrate 110 is formed of n-type silicon and functions as a gateelectrode. The insulating layer 120 is formed of silicon oxide which isthermally grown to a thickness of 300 nm on the substrate 110. Theorganic semiconductor layer 130 is formed of the compound of Formula 1.For example, the organic semiconductor layer may be formed to athickness of 50 nm by depositing the compound of Formula 2 under vacuum,or dissolving the compound of Formula 2 in a solution such aschlorobenzene and spin-coating the resulting solution. The electrodelayers 140 are formed by depositing 30 ml of gold on the organicsemiconductor layer under vacuum to serve as source and drainelectrodes.

When a novel thiazole-based organic semiconductor compound of thepresent invention is used for an organic thin film transistor, an on/offratio and thermal stability can be improved, and a solution process canbe adopted.

While exemplary embodiments of the present invention have been shown anddescribed in detail, it will be appreciated by those skilled in the artthat various changes can be made to these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined by the claims and their equivalents.

1. An organic thin film transistor, comprising: a silicon substrate; aninsulating layer formed of silicon oxide on the silicon substrate; anorganic semiconductor layer formed of a compound of Formula 1 on theinsulating layer; and an electrode layer formed on the organicsemiconductor layer,

wherein R1, R2, R3 and R4 are independently selected from a groupconsisting of hydrogen, a C1-C25 alkyl group, a C1-C25 alkoxy group, anda C6-C30 aryl group substituted with C1-C25 alkyl and alkoxy groups, andX and Y are substituents or substituent groups identically orindependently selected from a group consisting of

wherein Z1 is an independently single- or multi-substituted substituentselected from a group consisting of hydrogen, a hydroxyl group, asubstituted or unsubstituted methyl group, a carbonyl group, an aminogroup, a substituted or unsubstituted C1-C30 alkyl amino group, asubstituted or unsubstituted C1-C30 aryl amino group, a substituted orunsubstituted heteroaryl amino group, a cyano group, a halogen atom, asubstituted or unsubstituted C1-C30 alky group, a substituted orunsubstituted C3-C30 cycloalkyl group, a substituted or unsubstitutedC1-C30 alkoxy group, a substituted or unsubstituted C6-C30 aryl group, asubstituted or unsubstituted C6-C30 aryl alkyl group, a substituted orunsubstituted C2-C30 heteroaryl group, and a substituted orunsubstituted C2-C30 heterocyclic group; and R5, R6, R7 and R8 areselected from a group consisting of hydrogen, a C1-C25 ethylene oxidegroup, a C1-C25 alkyl group, a C1-C25 fluorinated alkyl group, a C1-C25alkoxy group, a C1-C25 alkylene oxide group, a C1-C25polydimethylsiloxane group, a C1-C25 alkylene sulfide group, and aC1-C25 ethylene sulfide group.
 2. The organic thin film transistoraccording to claim 1, wherein the organic semiconductor layer is formedby depositing the compound of Formula 1 under high vacuum.
 3. Theorganic thin film transistor according to claim 1, wherein the organicsemiconductor layer is formed by coating the compound of Formula 1dissolved in a solution.